The present invention relates to a method of plotting an equilibrium curve representative of the relation between the oxygen saturation value of a hemoglobin sample (such as blood) and a changing oxygen partial pressure level in a gas, in which the sample is placed in a measuring space, the saturation value being determined from the photometrically measured light absorption of the sample in the space. The invention also relates to an apparatus which is especially designed for carrying out the process according to the invention.
Known ways of plotting equilibrium curves of this type make use of the fact that the absorption properties of hemoglobin undergo changes in the visible light range and in the near IR and UV ranges as a function of the oxygenation of the hemoglobin, that is to say the amount of contained in it. In such a process the blood or other hemoglobin sample is placed as a thin film on a transparent film support so that it may be acted upon by the reaction gas on one side. It is furthermore known in this respect for the sample to be covered on the side acted upon by a the reaction gas by diaphragm which is transparent to light and permeable for the gases in question, such as diaphragm being made for example by PTFE (see K. Schmidt and K. H. Heuser, Z. Respiration 26 (1969), pages (16 to 34), which stabilizes the sample film and prevents the sample from drying up while its properties are being measured.
In a further process of the same general type (see German Auslegeschrift specification No. 2,504,771) an oxygen-containing gas is forced to pass at a low inlet speed into the deoxygenating gas and is agitated or swirled in it.
In all these known ways of testing the "affinity curve" of the oxygen there has been a trend of plotting the equilibrium curve while increasing the oxygen partial pressure. In fact in all cases the sample is initially completely deoxygenated and then at an increasing oxygen partial pressure the increase in the amount of the oxygen combined with the hemoglobin is measured by measuring the level of light absorption, or the relative difference in light absorption.
However, certain problems are encountered in connection with exactly measuring the level of the effective oxygen partial pressure in the measuring space and although the oxygen partial pressure may, as part of a first prior art process, be measured using an oxygen feeler electrode placed in the measuring space, the readings produced are not as exact as might be desired. In a further known measuring operation--the diffusion chamber process--no attempt is made for this reason to directly measure the oxygen partial pressure in the measuring space, and an indirect measuring operation for the oxygen partial pressure is based on the idea of such pressure increasing logarithmically in the measuring space when oxygen is diffusing into the measuring space through a diffusion resistance as for example a diffusion capillary (see H. Sick and K. Gersonde, Analytical Biochemistry 32 (1969), pages 362-376 and 47 (1972) pages 47 to 56). In the case of this process the time needed for a measuring cycle is dependent on the diffusion speed of the oxygen and may not be made arbitrarily as short as desired.